This apparatus is directed to an improved gain control system for a rotating transducer ultrasonic logging tool supported in a sonde and used in a well borehole to conduct ultrasonic logging. An ultrasonic testing device forms an ultrasonic wave which is transmitted directionally from a sonde in a well borehole. Particularly in an open hole (referring to an uncased well borehole), the device can generate tremendous amounts of data, the data flowing from the tool to the surface at the rate of about 1.5 megabytes per second. A typical present day ultrasonic tool fires repetitively, and each pulse on firing is directed differently in azimuth than the previous pulse transmitted by the device. There is a waiting interval while the ultrasonic echo is received back at the transducer which is used both for transmission and reception. The borehole is tested by a constantly rotating ultrasonic transducer operated in a pulsed mode. Presently, an image can be obtained with one inch vertical spacing, and consecutive pulses can be spaced as close as about one degree of azimuthal rotation. Indeed, even closer pulse spacing can be obtained.
As each transmitter pulse is formed, the pulse itself can be used as a timing marker serving as a reference and peak amplitude is also noted. The receiver system must respond dynamically using a gain control amplifier and handle the received signal. The received signal decays over several orders of magnitude. The receiver must have an automatic gain control (AGC) system so that the receiver is not overdriven or the data is otherwise lost because the receiver is providing insufficient gain. In other words, the gain for the receiver must be appropriate for the moment, and this gain typically has the form of a decaying gain correction factor, or decaying AGC signal. For instance, just a few microseconds after the ultrasonic pulse is transmitted, the AGC amplifier is switched so that gain is small because the return signal is quite large. In other instances, the return signal may be quite small and may be smaller by a few order of magnitude. The gain curve may also decay as an exponential. This AGC control signal must be applied to an AGC amplifier in the receiver circuitry to assure receiver gain control.
Even though a generalized AGC gain control curve can be devised, that is not sufficient information. For instance, there may be a vertical crack or fissure adjacent to the borehole which will show up on a few revolutions as the tool moves up the well borehole. This will cause a quite large signal at some point during the decay curve. If the gain control responds excessively, the large peak may be completely suppressed and will not be sufficiently amplified. This is an undesirable result. On the other hand, if the AGC amplifier responds sluggishly, the amplifier may be overdriven into saturation. The amplifier in the receiver must therefore be provided with appropriate gain control throughout the response time after firing of the transmitter.
In general measure, if the lithography of the well is known, the AGC gain can be reshaphed in advance. For instance, if it is known that the ultrasonic log is being formed in a sand formation as opposed to limestone, the general performance and response of the system can be known. To that end, preprogramming can be helpful to avoid overdriving or underdriving the receiver system. Programming will not capture signal dynamics such as those described above.
The present apparatus enables the wide ranging signals to be accomplished and to particularly provide an AGC control system which generates a sized or matched AGC control curve which takes into account the vagaries of the formation encountered and which also takes into account the data obtained from the previous full revolution of the ultrasonic transducer. In the latter event, it is assumed that adjacent revolutions will provide approximately similar dynamics of the data. Precise identity is not required; what is helpful, however, is the provision of an AGC control signal which is shaped somewhat by the prior revolution. One revolution however typically entails multiple pulses. In the preferred embodiment, the pulse identification number is readily handled as a digital word using the binary system. Accordingly, the full revolution (of rotation) is ideally divided into 256 or 512 increments. The latter gives a finer level of measurement or resolution. For each of the 256 or 512 angular positions, a transmitter pulse is formed and an AGC control curve is provided. One revolution (256 or 512 pulses) defines what is known as the A revolution. The next revolution, or the B revolution, serves as a good model for the gain for each pulse of the following or next revolution. In other words, the AGC signal for each pulse in one revolution is stored and is used as a model for the gain of the AGC amplifier during the next revolution. To be sure, while there will be differences from one revolution to the next, in general, the gain instructions and sequence for each of the pulses in a given revolution are quite similar to those in the prior revolution. As will be explained, for a particular pulse in a particular revolution, the prior revolution AGC signal serves as a useful and valuable predictor for the AGC control in the next following revolution at that particular pulse. In alternate embodiments, the prior N AGC settings (N is an integer) may be used to obtain an average.
The present apparatus is therefore summarized as a rotating ultrasonic transducer which is connected with a transmitter for periodic firing. It is fired by providing a procession of firing pulses to it from a transmitter. The transmitter is clocked so it operates in a timed sequence. One full revolution is divided into a specified number of transmitter pulses such as 512. These are spaced evenly in time so that they are transmitted in evenly spaced angular or rotational increments. With each rotation of the transducer, it forms pulses associated with firing. This enable synchronization of operation subsequently. The ultrasonic transducer is used to both transmit and receive, and the received signal is the return echo directed back to the transducer after transmission into the formations adjacent to the well borehole. The received signal normally decays over a period of time. An automatic gain control (AGC) signal is preferably used to control an adjustable gain amplifier connected to the transducer and operated during the receiver mode. The receiver is thus blanked or switched off during the firing of the transmitter and is switched on at some interval thereafter to receive the acoustic signal from the formation. This acoustic signal is received and digitized. This produces a remarkably large quantity of data, typically about 1.5 megabytes of data per second. The circuit thus includes an adjustable gain amplifier which connects with a synchronized analog to digital converter (ADC) and that forms the output which is delivered to a CPU so that it can be formatted and transmitted to the surface. The CPU is synchronized with the rotor so that the precise step in one revolution for each pulse is identified. In addition, there are duplicate A and B revolution signal buffers. While one is being filled with data, the data in the other is being used, and they then swap functions. Thus, each is filled and each is used for alternate revolutions of the transducer. Assuming that the transmitter is fired 512 times per revolution, this requires 512 addresses in each of the two buffers. The AGC control signal is input for each of the 512 pulse events. It is stored in one of the buffers while the other buffer is being used. In other words, 512 AGC signals for one revolution are written in one of the buffers in digital form and then the next revolution occurs while the next 512 AGC signals are written into that buffer. As will be seen, the data in first one buffer and then the other is used. Assume, as an example, that pulse 369 in a first revolution is associated with a particular gain for the AGC amplifier system. On the next revolution, pulse 369 causes interrogation of the data from the prior revolution at the common pulse location to obtain digital words representative of the particular gain setting; this utilizes the prior revolution corresponding data as a format for a subsequently occurring transmission. Clearly, the device is able to use the two buffers to store first one revolution and then the next, switching back and forth between the two buffers. Output of the data utilizes a downhole CPU which formats the measured received voltage as well as the digital word representative of the prior revolution to assure storage.